# MATH 166 Exam 3

**1. (5 points)** If
and
,and C = BA, find c_{12}:

**Answer:
**

d) The element in the first row, second column of the product is equal to the matrix product

of the first row of B times the second column of A. This gives us 2 * e + 5 * d.

**2. (5 points)**Alice borrows $15000 at 3% (compounded monthly) to buy a car, and makes

no payments on the loan for 5 years. How much does she owe (rounded to two decimal

places)?

**Answer:**

c) $17424.25172 is the answer given by the TVM solver when we enter n = 60, I% = 3,

PV = 15000, and C/Y = 12.

**3. (5 points)**Find the inverse of

**Answer:**

c) If we enter the matrix in the problem as A, then A

^{-1}gives us answer b)

**4. (5 points)
**

Bob borrowed $750 five months ago. If he now owes $776.25, what simple annual interest

rate did he borrow at?

**Answer:**

b) Bob owes $26.25 in interest for five months. A month’s interest would be

so a year’s interest would be $63. gives us the interest rate of 8.4%.

**5. (5 points)**If A is a 2× 3 matrix, B is a 2× 3 matrix, and C is a 3× 2 matrix, which of

the following is not a valid operation between A,B, and C?)

**Answer:**

d) Since the number of columns in the first matrix must equal the number of rows in the

second matrix, the B * A is not valid, since B has 3 columns and A has 2 rows.

**6. (5 points)**Using the matrices from the previous problem, which of the following is a

false statement?)

**Answer:**

b) Since A and C have different orders , it is not possible to add them .

**7. (5 points)**Paul buys a savings bond for $75 and in 5 years, it will mature to a value

of $100. Assuming quarterly compounded interest , what interest rate is he getting on his

savings bond?

**Answer:**

a) If we plug in n = 20,PV = 75, FV = −100, and C/Y = 4, we get 5.79522% as the

interest rate.

**8. (12 points)** Francine is investing $1000 from her
paycheck each year into real estate,

stocks, treasury bonds, and a bank account. She wants to puts half as much into
her

savings account as she does into the treasury bonds and stocks combined . She
estimates the

annual return on the investments to be 5% for real estate and savings, 3.5% for
treasury

bonds, and 8% for stocks. She wishes to achieve a 6.5% average annual return on
her

investments.

(a) Give a system of equations that represents the above information. (Use the
variables r

for real estate, s for stocks, t for treasury bonds, and a for the bank
account.)

(b) Solve the system of equations, and parameterize the answer in terms of a ,
the amount

in the bank account.

(c) Using the information above, what is the minimum and maximum she can put
into her

bank account?

(d) if she chooses to put $200 into her bank account each year, how much does
she put into

the other investments?

**Answer:**

(a) The information above can be summarized by writing equations (letting r=
real estate,

s = stocks, t = treasury bonds, and a = bank account):

r + t + s + a = 1000

a = .5t + .5s

.05r + .08s + .035t + .08a = .065 * 1000

(b) . r s t a

We type this in as the following matrix:

Using rref on this matrix, we get the following:

This tells us that and that

(c) Since Additionally, 1000 − 3a ≥ 0, so a ≤
333.33, giving us

250 ≤ a ≤ 333.33. Once we know boundaries for a, we can plug them to get
boundaries

for the other variables:

(d) Plugging in a = 200, we get r = $400, s = 466.66 and t
= −66.66 . The negative value

for t illustrates what happens when we give an a outside of our acceptable
boundaries

above.

**9. (6 points)** Using the following system of equations, either state that
there are no

solutions, that there are infinitely many solutions (with simplified equations
limiting the

solutions), or state the solution. Write all numbers as** fractions ** in your
answer.

x − 27 = 3z

4z + 5 = 6y

7x = 8z

**Answer:**

Putting the equations in standard form, we get x − 3z = 27,−6y + 4z = −5, and

7x − 8z = 0. Entering these in as the matrix

, and using rref, we get the

following answers: and

**
10. (8 points)** A single trip for a certain fishing boat consists of going to
the fishing area,

fishing for a number of days, and then returning to dock to sell the fish. The owner of the

boat has calculated the gas cost for each trip to be $300 to travel to the fishing area, $100

per day of fishing, and $1 per pound of fish carried plus $300 to return to the dock. If the

owner can sell fish at $3 per pound and catches 200 pounds of fish per day spent in the

fishing area, how many days must the boat remain in the fishing area to pay for its gas?

**Answer:**

Let f be the pounds of fish caught, and d be the number of days in the fishing area. Then

we can represent this problem as the following system of equations: f = 200d and 3f =

300 + 300 + f + 100d, meaning that $3 times the pounds of fish (the ship’s income from

fishing) must equal the amount of gas used ($300 for going out, $300 plus f for coming back,

and $100 per day). Putting these equations in simplified form gives us f − 200d = 0 and

2f − 100d = 600, giving us the following matrix:

Solving via rref gives us that f = 400, d = 2, so the ship
must spend two days fishing to

break even.

Alternatively, we can substitute f = 200d as we make the equations, giving the
single

equation 600d = 300 + 300 + 200d + 100d, which gives us the same answer.

**11. (8 points) **Solve for x,y, and z in the
following equation:

**Answer:**

Adding the matrices together gives us the following equations: x+1+24 = 27, 2+4z
= 14,

8y + 8 = 16, which gives us x = 2,y = 1, z = 3.

**12. (8 points)** A car manufacturer calculates their cost per car
manufactured in France

and India, and the number produced in each location in 2007 and 2008. The
production

matrix,P is given by:

The cost matrix, C, which gives the cost per unit produced
in each country, is given by:

(a) Find a meaningful product of the given matrices. (Be sure to clearly show
how you

multiplied them .)

(b) What information do the elements of the product give us? Be precise for each
element

in the product.

**Answer:**

(a) In order to have the labels match up correctly in the multiplication, we
need to change

C into a column matrix, giving us

Then we multiply, getting

(b) The first row will give us total costs for 2007,
covering both France and India. The

second row will give this same value for 2008. (The item in row 1 is French
production

for 2007 times unit cost plus Indian production for 2007 times unit cost. The
item in

the second row is the same, but for 2008 production.)

**13. (11 points)** A printing business takes out a 7
year loan at 4.9% interest (compounded

monthly) to purchase a new printer worth $220,000. The company paid 15% of the
value

up-front and borrowed the rest, making monthly payments.

**Answer:**

Entering in N = 12* 7, I = 4.9%, PV = 187000, FV = 0,P/Y = C/Y = 12 and solving
for

PMT gives us 2634.263095 as our monthly payment.

(a) Knowing our monthly payment, we set N = 12*5 and solve for FV to find the
remaining

principle after 5 years, which gives us 60106.46392

(b) We take the value owed from above (at the end of the fifth year is the same
as the

beginning of the 6th year). We need to calculate our interest each month and
subtract

that from our monthly payment to get the principal paid. Once we know the
principal

paid, we can find the principal remaining by subtraction.

Month | Principal at Start | Interest Paid | Principal Paid | Principal Remaining |

1 | $ 60106.46392 | $ 245.43472 | $ 2388.82836 | $ 57717.63555 |

2 | $ 57717.63555 | $ 235.58034 | $ 2398.58275 | $ 55319.05280 |

3 | $ 55319.05280 | $ 225.88613 | $ 2408.37696 | $ 52910.67584 |

**14. (12 points) **Joe is saving for retirement. He
deposits $200 every month in a bank

account which gives 3.5% interest, compounded monthly.

(a) What is the effective annual rate of interest on the account?

(b) After 20 years, how much is the account worth?

(c) After 20 years, how much interest has Joe earned?

**Answer:**

(a) There are 12 interest periods in the year, so the effective interest rate is
found by calculating

so the effective interest rate is 3.55669%.

(b) Using the TVM solver, we put in N = 12 * 20, I = 3.5%, PV = 0, PMT = −100,
and

P/Y = C/Y = 12. Solving for future value gives us 69373.85382.

(c) For an investment, Interest = FV - PV - PMT * N. Since the future value is
$69373.85382

and Joe has put in 12 * 200 * 20 = $48000, the total
interest is the difference of these , or

$21373.85382.

Prev | Next |